In extracting hydrocarbons from subterranean formations it is often necessary to increase permeability and flow of hydrocarbons out of formation and into a wellbore to be pumped to surface for production. One method of increasing permeability is to stimulate the formation through perforations formed in a casing running down the wellbore.
In some cases there are multiple hydrocarbon-bearing intervals in the formation and it is desirable to stimulate and then produce from each of these intervals. Commonly, the process is conducted by stimulating or fracturing one interval at a time. Such process is known as multi-stage stimulating.
In multi-stage stimulating it is desirable to isolate a particular interval to be stimulated, perforate the casing in that interval, stimulate or fracture the interval, then move to a next interval to be isolated, perforated and stimulated. These steps are achieved by running a tool string down into the casing of the wellbore, the tools string having an sealing device, a perforating device and may also include other devices for locating the tool string at the interval to be stimulated and anchoring the tools string to the casing to maintain position. Sealing devices can include bridge plugs, packers, ball sealers, sliding sleeves and straddle packers. These sealing devices may be hydraulically activated or mechanically activated from the surface. Perforating devices include explosive perforating charges fired from a perforating gun, high-pressure fluid perforators, sand jet perforating, burst disk or burst plug inserts among others. Anchoring devices commonly have slips with toothed surfaces for gripping against an inner surface of the casing to prevent axial and sometimes also radial movement of the tool string within the casing.
The tool string, also often called a bottom hole assembly (BHA), is typically run into the wellbore casing on coiled tubing or on jointed tubing. In such cases, fluid from the surface can be pumped through the coiled or jointed tubing into the tubing string to actuate isolating, anchoring and perforating devices. Alternately, the tubing can be mechanically manipulated at surfaces by pulling, pushing or turning, to actuate the various devices of the tool string.
One disadvantage of running a tool string on coiled or jointed tubing is that the inside diameter (ID) of the coiled or jointed tubing presents a reduced flow cross-sectional area than that of the full ID of casing itself. Furthermore, should fluid be flowed in the annulus between the outside diameter (OD) of the coiled or jointed tubing and the ID of the casing, this also presents reduced cross sectional area than the casing alone. Reduced cross sectional area is disadvantageous in that it increases frictional losses for fluids flowing through the narrower annulus. Surface pumps pumping the fluid into the narrow annulus are required to pump at much greater power to achieve the needed flow rates for stimulation and pumps often reach their maximum pumping power without reaching the needed flow rates.
U.S. Pat. No. 6,394,184 to Tolman et al. teaches a BHA for stimulating multiple intervals of a formation in which the BHA may be run on coiled tubing or jointed tubing. It also teaches one embodiment in which the BHA for stimulating multiple intervals can be run on a wireline. Tolman et al. teaches slips that are mechanically set using axial up and down movement of the tubing or wireline on which they are run. With respect to a wireline deployed BHA, Tolman also teaches a resettable inflatable packer that is connected to an electrical pump system that inflates or deflates the inflatable packer using wellbore fluid. The perforating devices of Tolman are either select-fire perforating guns or abrasive/erosive fluid-jet cutting tools.
There are a number of disadvantages to use of combined hydraulic and mechanical devices on the single BHA. For example, mechanically actuated devices needing axial movement of the string can interfere with proper locating and setting of other tools that do not need mechanical movement to set. As well, use of wellbore fluid to set any devices can introduce wellbore contaminants into the tool string increasing the risk of plugging the devices with wellbore debris and also increasing wear and damage.
Changing wellbore fluid properties like temperature and viscosity can also adversely affect actuation of hydraulic set tools.
It is therefore desirable to provide BHA's that do not limit fluid flow through the casing and which also reduce unpredictability of actuation.